This invention relates to an active matrix liquid crystal display device comprising a multiplicity of liquid crystal display elements, each of which is controlled by a thin film transistor having a gate layer, a semiconductor layer providing a channel region insulated from the gate layer by a gate insulation layer, and means for illuminating the display elements, the display elements comprising groups providing outputs in respective different colors.
Display devices of the above kind are well known. In a typical example the display device comprises a display panel having a pair of spaced, parallel, transparent substrates between which liquid crystal material is disposed. One substrate carries a row and column array of display element electrodes, sets of row and column address conductors and substantially identical thin film transistors, each of which is connected between a respective display element electrode and an associated pair of address conductors with its gate, source and drain terminals connected respectively to the row conductor, the column conductor, and the display element electrode. The other substrate carries a common electrode and a pattern of red, green and blue color filter elements in the form of a micro-filter array, the pattern being such that three adjacent display elements constitute a color triplet. The array thus consists of three interspersed groups of respectively colored (red, green and blue) display elements. The arrangement of the display elements within each triplet, and of the triplets with respect to their neighbours, can be varied. In operation, the array of display elements is illuminated with light and controlled via its TFT to modulate the light and provide a color output according to its filter elements.
Such a display device may be used as a projection display device in which intense white light is directed onto the substrate carrying the filter elements and is modulated appropriately by the individual display elements whose outputs are collectively projected onto a display screen by a projection lens.
In another known kind of liquid crystal color projection display device, the display elements are arranged as three separate groups, for example using three physically separate display panels, and each group is illuminated with light of a respective, different, primary color. The outputs of the individual display elements of each of the groups are combined and projected onto a display screen by a projection lens. In this case the display device can be regarded as comprising three monochromatic display panels providing a polychromatic display by combining their outputs. As such, individual color filter elements are not required.
Typically, the TFTs comprise amorphous silicon or polycrystalline silicon devices. These are fabricated on the one substrate by successive deposition and definition of thin film layers, for example comprising: a semiconductor layer of amorphous silicon or polycrystalline silicon, which provides a channel region; an insulative layer overlying the channel region and serving as a gate insulator, and a conductive layer overlying the channel region, and separated therefrom by the insulative layer, forming a gate. Source and drain contacts are provided at the ends of the channel region. An oxide or nitride passivation layer is normally used to cover this structure.
The semiconductor materials commonly used in TFTs, particularly amorphous silicon and polysilicon materials, have the undesirable property of being photoconductive. In an amorphous silicon TFT a metal gate can serve to reduce the amount of light reaching the semiconductor layer but not always adequately. In polysilicon TFTs, gates of doped polysilicon or silicide are often used which are not particularly effective in blocking light. Induced photocurrents caused by light reaching the channel region can influence significantly the off resistance of the TFT leading to a degradation in the performance of its associated display element. If large numbers of the TFTs are affected in this manner, then poor display quality results.
In order to alleviate this problem to some extent light shields have been incorporated in the device's structure to block light from the light source in the direction of TFTs. These light shields consist of regions of light absorbing material carried on the substrate facing the light source, between the filter elements if present, and overlying a respective TFT. In addition a metal pad may be provided on top of the TFT structure in cases where the channel region of the TFT is located closer to the liquid crystal than the gate.
Besides being inconvenient, the provision of light shields is not always an altogether effective solution.